The contributions of cognitive control to
episodic memory are not restricted to the building of memories, but
also extend to episodic retrieval––that is, the ability
to recollect contextual details surrounding a prior event or to detect
stimulus novelty or familiarity. Our efforts to understand the interaction
between cognitive control and declarative memory have included a focus
on identifying and characterizing the multi-component nature of PFC
contributions to retrieval with and without recollection.

In one line of investigation, we explored the neural
correlates of source memory, a form of remembering that depends
on recollecting a specific contextual detail about a past event.
Relative to memory without recollection, source retrieval is thought
to entail the recapitulation of neocortical representations present
during the encoding of an event, and to differentially depend on
strategic engagement of cognitive control. To test these hypotheses,
we used fMRI to index neural activation during retrieval with and without
source recollection. Our results revealed that (a) source recollection
is associated with the recapitulation of representations in perceptual
and conceptual processing regions that were engaged during initial encoding
of events, (b) such recapitulation effects are observed not only for
accurate remembrances, but also are present when we falsely recollect
having encountered a novel stimulus, and (c) although PFC processes are
differentially engaged during attempts to recollect, strategic engagement
of these processes is gated by an initial assessment of whether the memory
probe is familiar or novel [Kahn
et al., 2004]. Results from a recently
completed MEG study suggest that this familiarity-based gating of PFC
function may emerge through an interaction with lateral parietal regions
[Kahn et al., in prep; for a review of parietal cortex and episodic retrieval,
see Wagner et al., 2005]. Collectively, these findings inform neural
models of recollection, and address core debates surrounding cognitive
theories of remembering.

In a second line of investigation, we have begun to more precisely
specify the nature of the PFC control mechanisms engaged during
retrieval, focusing in on those that support “pre-retrieval processes” (e.g.,
representing and elaborating on retrieval cues in working memory)
and those that operate “post-retrieval” (e.g., monitoring
the products of retrieval as a basis for action) [Wagner,
Desmond et al., 1998; Dobbins
et al., 2002; Dobbins
et al., 2003; Dobbins & Wagner,
in press]. To this end, we first identified and characterized separable
PFC structures that are recruited depending on whether a retrieval
decision requires recollection of contextual details or an assessment
of stimulus familiarity [Dobbins
et al., 2003]. Our results revealed
distinct left lateral PFC and parietal structures that distinguished
attempted source recollection from judgments of relative stimulus
familiarity, with these retrieval orientation effects being independent
of retrieval outcome [for related strategic retrieval effects,
see Wagner, Desmond
et al., 1998]. Subsequently, we tested the
nature of the left PFC processes recruited during source recollection
[Dobbins et al.,
2002; Dobbins & Wagner,
in press]. Using conjunction analyses, we isolated multiple PFC
retrieval responses, some that appear correlated with “pre-retrieval” mechanisms
and some with “post-retrieval” mechanisms. In the former
class, activation in distinct PFC regions was related to semantic
analysis/cue specification (left anterior VLPFC), perceptual analysis/cue
specification (right VLPFC), and cue maintenance (left posterior
VLPFC). In the latter class, effects compatible with post-retrieval
monitoring were observed in multiple structures (dorsolateral and
frontopolar PFC).

Though many questions remain, and an integrated model delineating
how these putative mechanisms interact awaits specification, our investigations
constitute initial progress towards specifying how PFC-mediated cognitive
control processes support attempts to recollect the past. Our long-term
goal is to develop an integrated model of the cascade of neural and cognitive
events that give rise to episodic remembering.